1. Field of the Invention
The invention relates generally to oil production, and more particularly to systems and methods for cable-deployment of downhole equipment such as electric submersible pumps (ESP's).
2. Related Art
Oil is typically extracted from geological formations through the wells that extend far below the earth's surface. Often, the naturally existing pressure in the wells is insufficient to force the oil out of the wells. In this case, artificial lift systems such as ESP's are used to extract the oil from the wells. ESP's are also commonly utilized when operators want to increase the flow rate of the fluid being extracted, such as when the water cut (percentage of water versus oil) increases.
An ESP system includes a pump and a motor that are lowered into a producing region of the well. Typically, the pump is connected to a conduit (e.g., a tubing string) through which oil is pumped to the surface. This conduit is normally used to lower the ESP system into the well, and to retrieve the ESP from the well. A power source at the surface of the well is connected to the ESP motor via a power cable that is connected to the conduit. For example, the power cable may be banded to the exterior of the conduit. The power cable in this type of system normally does not bear any of the weight of the ESP. It should be noted that the term “load” is used herein to refer to weight.
Sometimes a well operator wishes to use a cable-deployed ESP system. Conventional cables, however, typically are not designed to support the weight of such a system, and do not normally have the tensile strength to support even their own weight in lengths over about 1000 feet. While cables have been designed to support the weight of a cable-deployed ESP system (including the ESP and the cable itself), their use in the hostile downhole environment has resulted in various problems that render these cables impractical or ineffective.
Conventional power cables for downhole equipment use annealed copper conductors to convey electrical power from the power source to the downhole equipment. While copper has excellent electrical conductivity, it has very low tensile yield strength. As a result, most prior art cables that have been designed for cable-deployed systems have relied on load-bearing structures within the cables that are separate from the electrical conductors.
For example, in one type of cable, several layers of steel wires are helically wound around a set of conventionally constructed conductors (typically insulated copper). The steel wires provide the load-bearing capability which allows the cable to support itself and the downhole equipment, while the conductors provide the capability to convey electrical power to the downhole equipment. When this type of cable is used, the helically wound steel wires have a tendency to shift along the length of the cable and bunch up. This is sometimes referred to as “bird-nesting”. The bunching of the load-bearing wires is exacerbated by changes in the temperature and tension along the cable, and it may be very difficult to deploy the cable in the well or retrieve the cable from the well without causing the bunching of the load-bearing wires. Moreover, the resulting bunching of the wires impedes deployment and retrieval of the cable and corresponding downhole equipment.
Another type of prior art cable that was designed to be load bearing employs two wire (e.g., steel) ropes to support the weight of the cable and downhole equipment. In this type of cable, conventional conductors (e.g., copper conductors covered by layers of insulation) are used to carry electrical power to the downhole equipment. The wire support ropes are positioned on opposite sides of the conductors, and the wire ropes and conductors are then “sandwiched” between molded plastic blocks. An outer armor wrap is then provided around this assembly. One of the most significant problems that arises with this type of cable is that the conductors tend to push through the insulation and sheathing when run into the well using an injector apparatus. The injector creates enormous lateral forces on the cable, which may in turn cause the conductors to be pushed through the insulation and sheathing, compromising the electrical integrity of the cable. As with the previously described cable, the tensile stresses on the cable and the temperature changes within the downhole environment can cause the different components of the cable to move relative to each other, which can lead to failure of the cable.